Pull the husks from the tomatillos and wash them under cool water until they no longer feel sticky. Cut them into quarters and put them into the work bowl of a food processor. Add the onion and garlic and process until smooth. Add the cilantro, jalapeno and lime juice and process until the jalapeno is finely chopped.
Scrape the mixture into a small saucepan. Season lightly with salt and bring to a boil over medium heat. Cook, stirring occasionally, until most of the liquid is boiled off and the salsa looks relish-y, about 15 minutes. Cool before using. The sauce can be refrigerated for up to 1 week. If refrigerated, you may want to add a little salt and/or lime juice to the salsa before serving.